As energy prices rise, gas separation membrane technology has been receiving increasing attention because of its potential for reducing the environmental impact and costs of industrial processes. Gas separation membranes offer a number of benefits over other gas separation technologies such as low energy consumption and process simplicity because separation by membranes do not require a phase transformation.
Polymeric membranes are the most widely used commercially for gas separations. These membranes are generally highly selective but poorly permeable. In comparison inorganic membranes are highly permeable but poorly selective. For better results hybrid materials in which polymers and ceramics are dispersed at a molecular level have been investigated as gas separation membranes. The resulting hybrid membranes can sometimes retain the desirable properties of each material, for example, the flexibility and selectivity of polymers and the thermal stability of ceramics. There are several studies that have attempted to introduce organic functionalities on inorganic membrane surfaces to increase interactions with a particular gas. The introduction of organic functional groups sometimes also contributes to the modification of the molecular structure of the material, which results in favorable selectivity or permeability.
Hybrid organic-inorganic membranes are generally prepared by sol-gel methods because the low temperature processing chemistry allows the introduction of organic molecules inside an inorganic network. Okui et al. [T. Okui et al., J. Sol-Gel Sci. Technol. 5(1995) 127] used the sol-gel and dip-coating technique to prepare a porous hybrid membrane composed of silica incorporating phenyl functional groups at 523 K on α-alumina porous substrates (mean pore size=0.7 μm). Tetramethoxysilane (TMOS, Si(OC3H3)4) and phenyltrimethoxysilane (PTMOS, C6H5—Si(OCH3)3) were the starting chemicals. Also, other functional groups such as methyl, propyl, 3-chloropropyl, 3,3,3-trifluoropropyl and octadecyl were introduced to study how changes in the affinity of these functional groups for permeating gases affects the permeation properties of the resulting materials. Among these organic functional groups, it was found that phenyl groups have the strongest affinity for CO2. Thus, silica membrane with phenyl groups had the best separation performance with values of permselectivity of 6.0 and 4.5 for CO2/N2 and CO2/CH4, with CO2 permeance of around 6.0×10−8 mol m−2 s−1 Pa−1 at 298 K (calculated from the reported permeability coefficient of 6.7 cc (STP) cm cm−2 s−1 cm Hg−1 and a thickness of 3-4 μm). It was also reported that the phenyl group in the hybrid membrane remained undamaged at 723 K. Smaihi et al. [M. Smaihi et al., J. Membr. Sci. 116(1996) 211] obtained an organic-inorganic gas separation membrane using a porous substrate and a thin organic-inorganic sol-gel layer as a permselective coating. These permselective layers were obtained by co-hydrolysis of tetramethoxysilane (TMOS) and phenyltrimethoxysilane (PTMOS) or diphenyldimethoxsilane (DPMOS, (C6H5)2—Si(OCH3)2), followed by calcination at 773 K. It was found that TMOS-DPMOS derived membranes exhibited higher permeance but selectivities in the Knudsen regime with CO2/N2=0.8, while TMOS-PTMOS derived membranes with contents of PTMOS above 70 mol % showed much higher selectivity, around 15 for CO2/N2. The CO2 permeance through a 54% PTMOS-TMOS derived membrane was 6.0×10−6 mol m−2 s−1 Pa−1 at 298 K while the selectivity of CO2/N2 was 4.0.
Another approach to enhance gas separation has been to prepare membranes containing large micropores by burning out functional groups attached to the silica. With plain silica membranes prepared by the decomposition of tetraethylorthosilicate (TEOS) [A. K. Prabhu, S. T. Oyama, J. Membr. Sci. 176 (2000) 233; S. Yan et al, Ind. Eng. Chem. Res. 33 (1994) 2096; H. Y. Ha et al. J. Membr. Sci. 85 (1993) 279], a microstructure is formed that allows the separation of small gaseous species like He and H2 from other gases. However, a silica membrane obtained by pyrolysis of phenyltriethoxysilane (PTES) or diphenyldiethoxysilane (DPDES) showed good CO2 separation [B. K. Sea et al. J. Membr. Sci., 130 (1997) 41]. This silica membrane was prepared on a porous alumina support first by chemical vapor deposition (CVD) using PTES or DPDES as the Si source at 773 K with the aid of evacuation, and then by calcination of the as-produced membrane in air at 673 K for 5 hours to remove the phenyl groups. The phenyl groups of PTES or DPDES remained unreacted under the CVD conditions at 773 K and decreased the silica cross-link density, thus leading to a loose amorphous structure after subsequent calcination. The DPDES-derived membrane had micropores larger than those of the TEOS-derived membrane, giving rise to a higher CO2 permeance of 8.1×10−8 mol m−2 s−1 Pa−1 at 298 K with a CO2/CH4 selectivity of 11, in comparison to a permeance of 3.6×10−10 mol m−2 s−1 Pa−1 and a selectivity of 0.66 for the TEOS-derived silica membrane.